1984 E23 733i Surging Idle When Warm

[JMWood]

Good day all,

1984 733i up in Canada. This car is in showroom condition. I bought from the off-lease buyer who owned it since 1988.

Car has a bit of a long crank, once it fires it runs perfectly for 1 to 2 minutes.
After it has warmed up the engine begins a surging idle, approx. 900rpm to 1500rpm, over and over indefinitely.
When it is put in drive it it feels that the idle speed is too high, wants to pull the car slightly.

My BMW specialist shop has spent 16 hours diagnosing without success.
- smoke test, no vacuum leaks
- disconnect O2 sensor, no change
- new idle control module, no change
- checked all wiring and pins on connectors, repaired a few suspect connections but no change


The technician wonders if a recently replaced temperature sensor could be the wrong unit, he says one is for below 0 and one is for above 40?
The weird thing is once with the new idle control module and once with the old, it ran perfectly but then acted up again on next restart.

If anyone has any helpful suggestions it would be greatly appreciated! Thanks in advance.

[YoungSensei]

It could be the coolant temperature sensor as you suggest. Pretty sure the ecu reads from it to determine idle speed while warming up, and uses it to ultimately decide "the car is warm". Using the wrong sensor, will open/close the signal at the wrong times to the ecu. Try jumping the two wires that go into the sensor together, and see if the idle settles down at all.

[Dushan]

I had one of those, many years ago. It had a similar problem and at the time the very knowledgeable, people at Beach Auto in Toronto (a BMW dealer) couldn’t quite figure out what was causing it. The suspect was the MAF, or whatever it is called. The box that has a flap to measure the amount of air going in the engine. It was most likely the spring that got weak. We replaced it with a used one and the problem went away.

[JMWood]

Great idea, will try this for sure and report back